The typical design of a microwave or mm-wave package has a ceramic substrate with a plurality of conductive microstrip transmission lines on the outside and between the layers of ceramic and vias to interconnect the transmission lines. An integrated circuit die is fixed, by means of an adhesive, in a central recess and is wire bonded to connect to the transmission lines. A continuous metal wall is positioned on and sealed to the perimeter of the ceramic substrate. The transmission lines do not pass under the metal wall. Instead, metal pins pass through holes in the wall. The pins are sealed to the wall and electrically isolated from it by a glass seal which fills the holes surrounding the pins. Transmission lines which need to be connected to the external environment are wire bonded to the end of the pins internal to the package. The package is then hermetically sealed by placing a metal lid on top of the wall and sealing the lid to the wall. In this design, signals are transmitted into and out of the package through the pins.
Sometimes there are advantages, both in cost and performance, to having the transmission lines pass under the wall. In such a design, there are no pins present. The transmission lines pass under a ceramic wall and connection is made from the ceramic substrate to the outside world by any one of a number of interconnection means, including solder or ribbon bonding.
Another technique provides interconnection between the inside and outside of the package by the use of vias which extend through the package substrate. Transmission lines are connected through the vias to conductive balls arrayed upon the bottom of the package. In this design, referred to as a ball grid array (BGA) package, the conductive balls are used to electrically connect the package to a microwave board.
There are performance problems with all of these scenarios. The performance characteristics of BGA packages makes them unsuitable for high power applications. The wire-bonding of the transmission lines to the pins of the typical design results in substantial losses which may not be acceptable. Similarly, passing transmission lines under the wall creates a discontinuity resulting in the reflection of a portion of the signal. As frequency increases, the discontinuity of the wall becomes more significant causing voltage standing wave ratio to deteriorate the performance to an unacceptable level.
One way in which the discontinuity caused by the wall has been compensated for has been to narrow the transmission line as it passes under the wall. However, this may pose manufacturing and performance problems.